Conventionally, MRI has been used to produce images by exciting the nuclei of hydrogen molecules (present in water protons) in the human body. However, it has been discovered that polarized noble gases can produce improved images of certain areas and regions of the body which have heretofore produced less than satisfactory images in this modality. Polarized Helium 3 (“3He”) and Xenon-129 (“129Xe”) have been found to be particularly suited for this purpose. See U.S. Pat. No. 5,545,396 to Albert et al., entitled “Magnetic Resonance Imaging Using Hyperpolarized Noble Gases”, the disclosure of which is hereby incorporated by reference herein as if recited in full herein.
In order to obtain sufficient quantities of the polarized gases necessary for imaging, hyperpolarizers are used to produce and accumulate polarized noble gases. Hyperpolarizers artificially enhance the polarization of certain noble gas nuclei (such as 129Xe or 3He) over the natural or equilibrium levels, i.e., the Boltzmann polarization. Such an increase is desirable because it enhances and increases the Magnetic Resonance Imaging (“MRI”) signal intensity, thereby potentially allowing physicians to obtain better images of many tissues and organs in the body.
Generally stated, in order to produce the hyperpolarized gas, the hyperpolarizer is configured such that the noble gas is blended with optically pumped alkali metal vapors such as rubidium (“Rb”). These optically pumped metal vapors collide with the nuclei of the noble gas and hyperpolarize the noble gas through a phenomenon known as “spin-exchange”. The “optical pumping” of the alkali metal vapor is produced by irradiating the alkali-metal vapor with circularly polarized light at the wavelength of the first principal resonance for the alkali metal (e.g., 795 nm for Rb). Generally described, the ground state atoms become excited, then subsequently decay back to the ground state. Under a modest magnetic field (10 Gauss), the cycling of atoms between the ground and excited states can yield nearly 100% polarization of the atoms in a few microseconds. This polarization is generally carried by the lone valence electron characteristics of the alkali metal. In the presence of non-zero nuclear spin noble gases, the alkali-metal vapor atoms can collide with the noble gas atoms in a manner in which the polarization of the valence electrons is transferred to the noble-gas nuclei through a mutual spin flip “spin-exchange”.
Conventionally, lasers have been used to optically pump the alkali metals. Various lasers emit light signals over various wavelength bands. In order to improve the optical pumping process for certain types of lasers (particularly those with broader bandwidth emissions), the absorption or resonance line width of the alkali metal can be broadened to more closely correspond with the particular laser emission bandwidth of the selected laser. This broadening can be achieved by pressure broadening, i.e., by using a buffer gas in the optical pumping chamber. Collisions of the alkali metal vapor with a buffer gas can lead to a broadening of the alkali's absorption bandwidth.
For example, it is known that the amount of polarized 129Xe which can be produced per unit time is directly proportional to the light power absorbed by the Rb vapor. Thus, polarizing 129Xe in large quantities generally takes a large amount of laser power. When using a diode laser array, the natural Rb absorption line bandwidth is typically many times narrower than the laser emission bandwidth. The Rb absorption range can be increased by using a buffer gas. Of course, the selection of a buffer gas can also undesirably impact the Rb-noble gas spin-exchange by potentially introducing an angular momentum loss of the alkali metal to the buffer gas rather than to the noble gas as desired. In any event, after the spin-exchange has been completed, the hyperpolarized gas is separated from the alkali metal prior to introduction into a patient.
Conventionally, gas-phase imaging has been possible hyperpolarized noble gases such as, but not limited to, 3He and 129Xe, each of which have been particularly useful in producing ventilation-driven (inhalation delivery) images of the lungs, a region where proton images have produced signal voids. For example, MRI images using gas-space-imaging techniques have been generated using hyperpolarized 129Xe gas. See Mugler III et al., MR Imaging and Spectroscopy Using Hyperpolarized 129Xe gas: Preliminary Human Results, 37 Magnetic Resonance in Medicine, pp. 809-815 (1997).